As shown in FIG. 1(a), an optical disk 2 comprises lands 6 and grooves 4 that contain recorded information. Optical disk drives read and write data from optical disks 2 by irradiating the optical disk with a laser. Due to the different reflection characteristics between the lands 6 and grooves 4, the content of the information written on the optical disk 2 may be determined by the different reflection characteristics.
The size of the lands 6 and grooves 4 on the optical disk 2 are fairly small. Thus, in order to properly read data from, and write data to, optical disks, the laser is focused on a particular track. The focusing and tracking of the laser often involves processes for generating and detecting tracking error signals.
In an example optical pickup module 1, as shown in FIG. 2, an optical pickup 3 is coupled to a sled 7 and a spring 5. As is known, the force on the spring 5 is controlled by a tracking control output signal. The controlled force on the spring 5 affects movement of theoptical pickup 3, thereby positioning the optical pickup 3 at a position dictated by the tracking control output signal. Since the movement of the optical pickup 3 is well known in the art, further discussion of the sled 7 movement is omitted here.
Several well-known processes for controlling the movement of the optical pickup 3 are the push-pull method and the three-beam method. Since both of these processes are well-known in the art, only a cursory discussion of these processes is provided here.
As illustrated in FIG. 1(b), in the push-pull process, when the laser beam is focused on a track 8, the difference between the light returned from the disc on both sides of the track 10, 12 may be measured. When the laser beam is centered exactly over the track 8, the difference between the light reflected from one side 10 of the disc and the light reflected from the other side 12 of the disc is zero. However, when the beam is off center, the push-pull tracking then becomes positive or negative.
In the three-beam method, the laser beam is divided into three beams, one of which follows the track under consideration (i.e., the central track), while the other two are focused on adjacent tracks (i.e., outriggers), immediately before and after the desired track. Any movement of the central track away from its desired position will cause an increase in the signal from one of the outriggers and, simultaneously, a decrease in the signal from the other outrigger. A comparison of the two outrigger signals provides sufficient information for a track-following servo.
As is known in the art, the tracking error signal is generated during the tracking process for indicating the deviation between a track position and the position of the optical pickup 3. However, when the offset is included in the tracking error signal, a deviation corresponding to the offset is induced between the track position and the position of the optical pickup 3. Moreover, in an optical disk drive adapted for counting the number of tracks that laser beams traverse based on the tracking error signal, if any offset exists in the tracking error signal as described above, it is impossible to precisely count the number of tracks. Thus, the process of removing the offset from the tracking error signal is carried out before reproducing/recording the information recorded on the disk. (i.e. compensating the tracking error signal)
When the push-pull method and the three-beam method are combined, the offset of the tracking error signal (TE) is a function of a main-push-pull-signal offset (MPP), which represents the signal from the central track, a sub-push-pull-signal offset (SPP), which represents the signal from one of the outriggers, and an offset correction factor (A). In other words, the TE may be represented as:TE=MPP+(A*SPP)  [Eq. 1].
As shown in FIG. 3, a conventional optical disk drive produces TE by introducing a tracking control output signal (TC) 9, which is typically a sine wave. The offset correction factor (A) is recursively altered during consecutive time intervals 25, 27, 29, thereby providing an iterative approach to compensating TE. Unfortunately, the shape of the sine wave sometimes produces erroneous TE.
Thus, a heretofore-unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.